1. Field of the Invention
This invention relates generally to color video display apparatus, such as color television receivers or monitors for color video recording and reproducing apparatus (VTR), and more particularly is directed to an improved hue correction circuit responsive to a vertical interval reference (VIR) signal included in the color video signal.
2. Description of the Prior Art
The chrominance signal in a broadcast composite color video signal is subject to phase shift or distortions in the course of transmission of the video signal. As a result of the foregoing phase shift or distortion, the color image presented to a viewer on a video display apparatus may have hue distortions, that is, the displayed images may vary in hue from the original picture.
Further, in a color video tape recorder or the like, in which a so-called "color-under" or "lower-frequency-converted recording method" is used to convert the chrominance signal to a lower frequency range for recording, the FM luminance signal operates as a biasing voltage for the converted chrominance component in the recorded composite signal. As a result of the biasing by the luminance component, upon reproduction and reconversion of the chrominance signal to the original frequency range, the chrominance signal is subject to phase distortion with changes in the luminance of the recorded signal. Thus, the reproduced video signal can produce a video display in which the image suffers from hue distortion.
It is known to include in a color television receiver a circuit for automatically adjusting the hue to correct for phase errors. The automatic hue adjustment employs a vertical interval reference (VIR) signal which is part of the transmitted or broadcast color video signal. The VIR signal is, in accordance with the accepted standard, generated as part of the television signal during the vertical blanking interval and more particularly usually during the nineteenth horizontal line interval of the vertical blanking interval). Although the VIR signal includes luminance and chrominance reference portions or components that would otherwise be visible, the VIR signal occurs when the electron beam or beams of the picture tube scan a region of the screen that is not visible so as to avoid interference with the picture being viewed.
The chrominance and luminance reference portions of the VIR signal have been selected to correspond in amplitude to values that would be present during transmission of a color video signal representing human skin tones. Thus, the chrominance reference portion of the VIR signal is superimposed on a luminance level of 70 IRE units, which level corresponds to the approximate brightness of average skin tones of a person of the white European or caucasian race. The relation of the VIR signal to skin tones has been selected because a viewer will be aware of approximately the proper hue of skin tones, although the viewer may have no way of knowing the proper hue of other objects in the television picture.
However, the VIR signal has been designed to correspond to the correct skin tone for persons of European (white) or Caucasian race.
Existing television receivers responsive to the VIR signal present an image in which skin tones of persons of the European (white) or Caucasian race have the correct hue, but the correct hue is not realized in respect to the skin tones of other races which have different levels. For example, skin tone levels for persons of the Asian (yellow) races may be between 40-60 IRE units, and for persons of the African (black) races may be perhaps only 20-40 IRE units. Existing hue correction circuits carry out hue-correction only at the so called average skin tone level of 70 IRE, and do not carry out hue correction over the entire range of brightness levels. Therefore prior art hue correction circuits do not present a video image with fully corrected skin tones for images of persons of non-European races.